The prior art provides various examples of piezoelectric transducers. Examples of such piezoelectric transducers are disclosed in U.S. Pat. Nos. 6,140,740; 6,064,746; 5,956,292; 5,751,827; 5,633,552; 4,654,554, and 4,979,219. In many cases, the known piezoelectric vibrating plate comprises a single thin metal sheet on one or both sides of which is or are laminated a piezoelectric sheet or sheets consisting of a round thin piece of 20 to 30 mm in diameter. A conventional piezoelectric speaker has a construction in which a vibrating film or sheet is stretched on a frame while being applied tension and a plurality of piezoelectric ceramics are directly stuck on the film. However, ceramic is so fragile that it is very difficult to make thin sheet and also it is not economical in terms of mass production with on-chip circuitry for signal conditioning.
Recently, there has been increasing interest in micromachined acoustic transducers based on the following advantages: size miniaturization with extremely small weight, potentially low cost due to the batch processing, possibility of integrating transducers and circuits on a single chip, lack of transducer “ringing” due to small diaphragm mass. Especially, these advantages make the micromachined acoustic transducers, such as microspeaker and microphone attractive in the applications for personal communication systems, multimedia systems, hearing aid and so on.
Micromachined acoustic transducers are provided with a thin diaphragm by deposition system and several diaphragm materials that must be compatible with high temperature semiconductor process, such as low stress silicon nitride and silicon have been applied as diaphragm. However, micromachined acoustic transducers made by these conventional diaphragm materials suffer from a relatively low output pressure and sensitivity, which are mainly because of the high stiffness and low deflection of these diaphragm materials in case of transducers application. So, in some cases, a conventional piezoelectric speaker used fiber reinforced epoxy, polyester, or ABS resin diaphragm in order to increase the deflection of diaphragm reported in U.S. Pat. No. 5,751,827.
In order to implement the micromachined microspeaker transducers with competitive performance with conventional microspeaker, it is necessary to find the new diaphragm materials that have large deflection with small driving voltage and compatibility with semiconductor process at the same time. Also, proper material and technique should be investigated to cause large deflection of diaphragm.
For the foregoing reasons, there is a need for a micromachined piezoelectric microspeaker which has a new diaphragm materials that have large deflection with small driving voltage and compatibility with semiconductor process at the same time.